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1.  The Orphan Nuclear Receptor Nur77 Is a Determinant of Myofiber Size and Muscle Mass in Mice 
Molecular and Cellular Biology  2015;35(7):1125-1138.
We previously showed that the orphan nuclear receptor Nur77 (Nr4a1) plays an important role in the regulation of glucose homeostasis and oxidative metabolism in skeletal muscle. Here, we show using both gain- and loss-of-function models that Nur77 is also a regulator of muscle growth in mice. Transgenic expression of Nur77 in skeletal muscle in mice led to increases in myofiber size. Conversely, mice with global or muscle-specific deficiency in Nur77 exhibited reduced muscle mass and myofiber size. In contrast to Nur77 deficiency, deletion of the highly related nuclear receptor NOR1 (Nr4a3) had minimal effect on muscle mass and myofiber size. We further show that Nur77 mediates its effects on muscle size by orchestrating transcriptional programs that favor muscle growth, including the induction of insulin-like growth factor 1 (IGF1), as well as concomitant downregulation of growth-inhibitory genes, including myostatin, Fbxo32 (MAFbx), and Trim63 (MuRF1). Nur77-mediated increase in IGF1 led to activation of the Akt-mTOR-S6K cascade and the inhibition of FoxO3a activity. The dependence of Nur77 on IGF1 was recapitulated in primary myoblasts, establishing this as a cell-autonomous effect. Collectively, our findings identify Nur77 as a novel regulator of myofiber size and a potential transcriptional link between cellular metabolism and muscle growth.
PMCID: PMC4355536  PMID: 25605333
2.  Dietary Cholesterol Promotes Adipocyte Hypertrophy and Adipose Tissue Inflammation in Visceral, But Not Subcutaneous, Fat in Monkeys 
Excessive caloric intake is associated with obesity and adipose tissue dysfunction. However, the role of dietary cholesterol in this process is unknown. The aim of this study was to determine whether increasing dietary cholesterol intake alters adipose tissue cholesterol content, adipocyte size, and endocrine function in nonhuman primates.
Approach and Results
Age-matched, male African Green monkeys (n=5 per group) were assigned to one of three diets containing 0.002 (Lo), 0.2 (Med) or 0.4 (Hi) mg cholesterol/Kcal. After 10 weeks of diet feeding, animals were euthanized for adipose tissue, liver, and plasma collection. With increasing dietary cholesterol, free cholesterol (FC) content and adipocyte size increased in a step-wise manner in visceral, but not subcutaneous fat, with a significant association between visceral adipocyte size and FC content (r2=0.298; n=15; p=0.035). In visceral fat, dietary cholesterol intake was associated with: 1) increased pro-inflammatory gene expression and macrophage recruitment, 2) decreased expression of genes involved in cholesterol biosynthesis and lipoprotein uptake, and 3) increased expression of proteins involved in FC efflux.
Increasing dietary cholesterol selectively increases visceral fat adipocyte size, FC and macrophage content, and proinflammatory gene expression in nonhuman primates. Visceral fat cells appear to compensate for increased dietary cholesterol by limiting cholesterol uptake/synthesis and increasing FC efflux pathways.
PMCID: PMC4140949  PMID: 24969772
adipocyte; cholesterol; dietary cholesterol; white adipose tissue; inflammation; African Green monkey; nonhuman primate
3.  Transgenic Expression of Dominant-Active IDOL in Liver Causes Diet-Induced Hypercholesterolemia and Atherosclerosis in Mice 
Circulation research  2014;115(4):442-449.
The E3 ubiquitin ligase IDOL triggers lysosomal degradation of the LDL receptor. The tissue-specific effects of the IDOL pathway on plasma cholesterol and atherosclerosis have not been examined.
Given that the liver is the primary determinant of plasma cholesterol levels, we sought to examine the consequence of effect of chronic liver-specific expression of a dominant active form of IDOL in mice.
Methods and Results
We expressed a degradation-resistant, dominant-active form of IDOL (sIDOL) in C57Bl/6J mice from the liver-specific albumin promoter (L-sIDOL transgenics). L-sIDOL mice were fed a Western diet for 20 or 30 weeks and then analyzed for plasma lipid levels and atherosclerotic lesion formation. L-sIDOL mice showed dramatic reductions in hepatic LDLR protein and increased plasma LDL cholesterol levels on both chow and Western diets. Moreover, L-sIDOL mice developed marked atherosclerotic lesions when fed a Western diet. Lesion formation in L-sIDOL mice was more robust than in ApoE*3 Leiden mice and did not require the addition of cholate to the diet. Western diet-fed L-sIDOL mice had elevated expression of LXR target genes and pro-inflammatory genes in their aortas.
Liver-specific expression of dominant-active IDOL is associated with hypercholesterolemia and a marked elevation in atherosclerotic lesions. Our results show that increased activity of the IDOL pathway in the liver can override other LDLR regulatory pathways leading to cardiovascular disease. L-sIDOL mice are a robust, dominantly-inherited, diet-inducible model for the study of atherosclerosis.
PMCID: PMC4119488  PMID: 24935961
Atherosclerosis; lipoprotein metabolism; mouse model; E3 ubiquitin ligase; LDL cholesterol; nuclear receptor
4.  LXRs link metabolism to inflammation through Abca1-dependent regulation of membrane composition and TLR signaling 
eLife  null;4:e08009.
The liver X receptors (LXRs) are transcriptional regulators of lipid homeostasis that also have potent anti-inflammatory effects. The molecular basis for their anti-inflammatory effects is incompletely understood, but has been proposed to involve the indirect tethering of LXRs to inflammatory gene promoters. Here we demonstrate that the ability of LXRs to repress inflammatory gene expression in cells and mice derives primarily from their ability to regulate lipid metabolism through transcriptional activation and can occur in the absence of SUMOylation. Moreover, we identify the putative lipid transporter Abca1 as a critical mediator of LXR's anti-inflammatory effects. Activation of LXR inhibits signaling from TLRs 2, 4 and 9 to their downstream NF-κB and MAPK effectors through Abca1-dependent changes in membrane lipid organization that disrupt the recruitment of MyD88 and TRAF6. These data suggest that a common mechanism-direct transcriptional activation-underlies the dual biological functions of LXRs in metabolism and inflammation.
eLife digest
Inflammation is a normal part of the immune response to infection or tissue damage. However, increased inflammation has been linked to diseases such as obesity, diabetes and atherosclerosis (in which the walls of the arteries become hardened). These same diseases have also been linked to problems with the production or breakdown of fatty molecules, such as cholesterol.
Transcription factors are proteins that bind to DNA to control gene expression. A transcription factor called LXR regulates the production and breakdown of cholesterol in response to changing levels of cholesterol in the body. LXR has also been shown to inhibit inflammatory responses, but previous studies suggested that these two actions of LXR are independent of each other.
Ito et al. have now challenged these findings by showing that LXR inhibits inflammation via changes in the metabolism of cholesterol and other fatty molecules. The experiments used genetically engineered immune cells, called macrophages, and mice to show that activating LXR causes cholesterol molecules to move between the membranes in a cell. This in turn leads to changes in the signals sent by proteins found at the cell surface, and eventually to a reduction of inflammation responses.
Future work will focus on better understanding the link between LXR's effects on metabolism and inflammation in models of human diseases such as diabetes and atherosclerosis.
PMCID: PMC4517437  PMID: 26173179
nuclear receptor; LXR; lipid metabolism; Toll-like receptor; mouse
5.  The GPIHBP1–LPL complex is responsible for the margination of triglyceride-rich lipoproteins in capillaries 
Cell metabolism  2014;19(5):849-860.
Triglyceride-rich lipoproteins (TRLs) undergo lipolysis by lipoprotein lipase (LPL), an enzyme that is transported to the capillary lumen by an endothelial cell protein, GPIHBP1. For LPL-mediated lipolysis to occur, TRLs must bind to the lumen of capillaries. This process is often assumed to involve heparan sulfate proteoglycans (HSPGs), but we suspected that TRL margination might instead require GPIHBP1. Indeed, TRLs marginate along the heart capillaries of wild-type but not Gpihbp1−/− mice, as judged by fluorescence microscopy, quantitative assays with infrared-dye–labeled lipoproteins, and EM tomography. Both cell culture and in vivo studies showed that TRL margination depends on LPL bound to GPIHBP1. Of note, the expression of LPL by endothelial cells in Gpihbp1−/− mice did not restore defective TRL margination, implying that the binding of LPL to HSPGs is ineffective in promoting TRL margination. Our studies show that GPIHBP1-bound LPL is the main determinant of TRL margination.
PMCID: PMC4143151  PMID: 24726386
6.  Retinoid X receptor α attenuates host antiviral response by suppressing type I interferon 
Nature communications  2014;5:5494.
The retinoid X receptor α (RXRα), a key nuclear receptor in metabolic processes, is down-regulated during host antiviral response. However, the roles of RXRα in host antiviral response are unknown. Here we show that RXRα overexpression or ligand activation increases host susceptibility to viral infections in vitro and in vivo, while Rxra −/− or antagonist treatment reduces infection by the same viruses. Consistent with these functional studies, ligand activation of RXR inhibits the expression of antiviral genes including type I interferon (IFN) and Rxra −/− macrophages produce more IFNβ than WT macrophages in response to polyI:C stimulation. Further results indicate that ligand activation of RXR suppresses the nuclear translocation of β-catenin, a co-activator of IFNβ enhanceosome. Thus, our studies have uncovered a novel RXR-dependent innate immune regulatory pathway, suggesting that the downregulation of RXR expression or RXR antagonist treatment benefits host antiviral response, whereas RXR agonist treatment may increase the risk of viral infections.
PMCID: PMC4380327  PMID: 25417649
7.  Lpcat3-dependent production of arachidonoyl phospholipids is a key determinant of triglyceride secretion 
eLife  null;4:e06557.
The role of specific phospholipids (PLs) in lipid transport has been difficult to assess due to an inability to selectively manipulate membrane composition in vivo. Here we show that the phospholipid remodeling enzyme lysophosphatidylcholine acyltransferase 3 (Lpcat3) is a critical determinant of triglyceride (TG) secretion due to its unique ability to catalyze the incorporation of arachidonate into membranes. Mice lacking Lpcat3 in the intestine fail to thrive during weaning and exhibit enterocyte lipid accumulation and reduced plasma TGs. Mice lacking Lpcat3 in the liver show reduced plasma TGs, hepatosteatosis, and secrete lipid-poor very low-density lipoprotein (VLDL) lacking arachidonoyl PLs. Mechanistic studies indicate that Lpcat3 activity impacts membrane lipid mobility in living cells, suggesting a biophysical basis for the requirement of arachidonoyl PLs in lipidating lipoprotein particles. These data identify Lpcat3 as a key factor in lipoprotein production and illustrate how manipulation of membrane composition can be used as a regulatory mechanism to control metabolic pathways.
eLife digest
Living cells are surrounded by a membrane that forms a barrier between the cell and its external environment. This membrane is largely made up of a variety of molecules known as lipids. The particular lipid molecules found in a cell membrane strongly influence its mobility, flexibility and other physical properties.
The liver and intestine can package lipids gained from the diet into molecules called lipoproteins, which are released into the bloodstream for use by the body. An enzyme known as Lpcat3 is found in high levels in the liver and intestine and it appears to be involved in the production of lipoproteins. Altering the amount of Lpcat3 in cells can change the types of lipids found in the cell membranes, connected to the production of lipoproteins.
Rong et al. studied newborn mice that were missing the Lpcat3 protein in either the liver or intestine. Mice lacking Lpcat3 in the intestine had higher levels of lipids inside their intestine cells and grew more slowly than normal mice. Mice lacking Lpcat3 in the liver also accumulated lipids in their cells and their bloodstream had lower levels of lipids that contain a molecule called arachidonic acid than normal mice. Further experiments showed that the loss of Lpcat3 reduces the ability of lipids to move within the cell membrane.
The experiments show that Lpcat3 plays a key role in attaching arachidonic acid to membrane lipids to promote the release of lipoproteins into the bloodstream. Rong et al.'s findings reveal that changing the type of lipids in the cell membrane plays an important role in regulating the levels of lipids in the blood.
PMCID: PMC4400582  PMID: 25806685
phospholipid; lipoprotein; nuclear receptor; mouse
8.  Progesterone Receptor in the Vascular Endothelium Triggers Physiological Uterine Permeability Pre-implantation 
Cell  2014;156(3):549-562.
Vascular permeability is frequently associated with inflammation and triggered by a cohort of secreted permeability factors such as VEGF. Here we show that the physiological vascular permeability that precedes implantation is directly controlled by progesterone receptor (PR) and is independent of VEGF. Both global and endothelial-specific deletion of PR block physiological vascular permeability in the uterus whereas misexpression of PR in the endothelium of other organs results in ectopic vascular leakage. Integration of an endothelial genome-wide transcriptional profile with ChIP-sequencing revealed that PR induces a NR4A1 (Nur77/TR3)-dependent transcriptional program that broadly regulates vascular permeability in response to progesterone. Silencing of NR4A1 blocks PR-mediated permeability responses indicating a direct link between PR and NR4A1. This program triggers concurrent suppression of several junctional proteins and leads to an effective, timely and venous-specific regulation of vascular barrier function that is critical to embryo implantation.
PMCID: PMC3985399  PMID: 24485460
9.  Feedback regulation of cholesterol uptake by the LXR-IDOL-LDLR axis 
Inducible Degrader Of the Low-density lipoprotein receptor (IDOL) is an E3 ubiquitin ligase that mediates the ubiquitination and degradation of the low-density lipoprotein receptor (LDLR). IDOL expression is controlled at the transcriptional level by the cholesterol-sensing nuclear receptor LXR. In response to rising cellular sterol levels, activated LXR induces IDOL production, thereby limiting further uptake of exogenous cholesterol through the LDLR pathway. The LXR–IDOL–LDLR mechanism for feedback inhibition of cholesterol uptake is independent of and complementary to the SREBP pathway. Since the initial description of the LXR–IDOL pathway, biochemical studies have helped to define the structural basis for both IDOL target recognition and LDLR ubiquitin transfer. Recent work has also suggested links between IDOL and human lipid metabolism.
PMCID: PMC4280256  PMID: 22936343
10.  Resistance to Diet-Induced Obesity in Mice with Synthetic Glyoxylate Shunt 
Cell metabolism  2009;9(6):525-536.
Given the success in engineering synthetic phenotypes in microbes and mammalian cells, constructing non-native pathways in mammals has become increasingly attractive for understanding and identifying potential targets for treating metabolic disorders. Here we introduced the glyoxylate shunt into mouse liver to investigate mammalian fatty acid metabolism. Mice expressing the shunt showed resistance to diet-induced obesity on a high fat diet despite similar food consumption. This was accompanied by a decrease in total fat mass, circulating leptin levels, plasma triglyceride concentration, and a signaling metabolite in liver, malonyl-CoA, that inhibits fatty acid degradation. Contrary to plants and bacteria, in which the glyoxylate shunt prevents the complete oxidation of fatty acids, this pathway when introduced in mice increases fatty acid oxidation such that resistance to diet-induced obesity develops. This work suggests that using non-native pathways in higher organisms to explore and modulate metabolism may be a useful approach.
PMCID: PMC4277884  PMID: 19490907
11.  Palmoplantar keratoderma along with neuromuscular and metabolic phenotypes in Slurp1-deficient mice 
Mutations in SLURP1 cause mal de Meleda, a rare palmoplantar keratoderma (PPK). SLURP1 is a secreted protein that is expressed highly in keratinocytes but has also been identified elsewhere (e.g., spinal cord neurons). Here, we examined Slurp1-deficient mice (Slurp1−/−) created by replacing exon 2 with β-gal and neo cassettes. Slurp1−/− mice developed severe PPK characterized by increased keratinocyte proliferation, an accumulation of lipid droplets in the stratum corneum, and a water barrier defect. In addition, Slurp1−/− mice exhibited reduced adiposity, protection from obesity on a high-fat diet, low plasma lipid levels, and a neuromuscular abnormality (hind limb clasping). Initially, it was unclear whether the metabolic and neuromuscular phenotypes were due to Slurp1 deficiency because we found that the targeted Slurp1 mutation reduced the expression of several neighboring genes (e.g., Slurp2, Lypd2). We therefore created a new line of knockout mice (Slurp1X−/− mice) with a simple nonsense mutation in exon 2. The Slurp1X mutation did not reduce the expression of adjacent genes, but Slurp1X−/− mice exhibited all of the phenotypes observed in the original line of knockout mice. Thus, Slurp1 deficiency in mice elicits metabolic and neuromuscular abnormalities in addition to PPK.
PMCID: PMC4214150  PMID: 24499735
12.  Reciprocal Regulation of Hepatic and Adipose Lipogenesis by Liver X Receptors in Obesity and Insulin Resistance 
Cell metabolism  2013;18(1):106-117.
Liver X receptors (LXRs) regulate lipogenesis and inflammation, but their contribution to the metabolic syndrome is unclear. We show that LXR signaling is required for key aspects of the metabolic syndrome in obese mice. LXRαβ-deficient-ob/ob (LOKO) mice remain obese, but show reduced hepatic steatosis and improved insulin sensitivity compared to ob/ob mice. Impaired hepatic lipogenesis in LOKO mice is accompanied by reciprocal increases in adipose lipid storage, reflecting tissue-selective effects of LXR on the SREBP, PPARγ, and ChREBP lipogenic pathways. LXRs are essential for obesity-driven SREBP-1c and ChREBP activity in liver, but not fat. Furthermore, loss of LXRs in obesity promotes adipose PPARγ and ChREBP-β activity, leading to improved insulin sensitivity. LOKO mice also exhibit defects in beta-cell mass and proliferation despite improved insulin sensitivity. Our data suggest that sterol sensing by LXRs in obesity is critically linked with lipid and glucose homeostasis and provide insight into complex relationships between LXR and insulin signaling.
PMCID: PMC4089509  PMID: 23823481
Nuclear receptor; liver X receptor (LXR); peroxisome proliferator-activated receptor (PPAR); carbohydrate response element binding protein (ChREBP); insulin resistance; diabetes; obesity; metabolic syndrome; hepatic steatosis; insulin signaling
13.  Lipins, Lipinopathies, and the Modulation of Cellular Lipid Storage and Signaling 
Progress in lipid research  2013;52(3):10.1016/j.plipres.2013.04.001.
Members of the lipin protein family are phosphatidate phosphatase (PAP) enzymes, which catalyze the dephosphorylation of phosphatidic acid to diacylglycerol, the penultimate step in TAG synthesis. Lipins are unique among the glycerolipid biosynthetic enzymes in that they also promote fatty acid oxidation through their activity as co-regulators of gene expression by DNA-bound transcription factors. Lipin function has been evolutionarily conserved from a single ortholog in yeast to the mammalian family of three lipin proteins—lipin-1, lipin-2, and lipin-3. In mice and humans, the levels of lipin activity are a determinant of TAG storage in diverse cell types, and humans with deficiency in lipin-1 or lipin-2 have severe metabolic diseases. Recent work has highlighted the complex physiological interactions between members of the lipin protein family, which exhibit both overlapping and unique functions in specific tissues. The analysis of “lipinopathies” in mouse models and in humans has revealed an important role for lipin activity in the regulation of lipid intermediates (phosphatidate and diacylglycerol), which influence fundamental cellular processes including adipocyte and nerve cell differentiation, adipocyte lipolysis, and hepatic insulin signaling. The elucidation of lipin molecular and physiological functions could lead to novel approaches to modulate cellular lipid storage and metabolic disease.
PMCID: PMC3830937  PMID: 23603613
14.  Leading by example: pastors, mentors, physician-scientists, and the ASCI 
The Journal of Clinical Investigation  2014;124(7):2816-2821.
PMCID: PMC4071402  PMID: 24983420
15.  LXRs regulate ER stress and inflammation through dynamic modulation of membrane phospholipid composition 
Cell metabolism  2013;18(5):685-697.
The fatty acyl composition of phospholipids determines the biophysical character of membranes and impacts the function of membrane proteins. Here we define a nuclear receptor pathway for the dynamic modulation of membrane composition in response to changes in cellular lipid metabolism. Ligand activation of LXR preferentially drives the incorporation of polyunsaturated fatty acids into phospholipids through induction of the remodeling enzyme Lpcat3. Promotion of Lpcat3 activity ameliorates ER stress induced by saturated free fatty acids in vitro or by obesity and hepatic lipid accumulation in vivo. Conversely, Lpcat3 knockdown in liver exacerbates ER stress and inflammation. Mechanistically, Lpcat3 modulates inflammation both by regulating c-Src and JNK kinase activation through changes in membrane composition and by affecting substrate availability for inflammatory mediator production. These results outline an endogenous mechanism for the preservation of membrane homeostasis during lipid stress and identify Lpcat3 as an important mediator of LXRs effects on metabolism.
PMCID: PMC3889491  PMID: 24206663
16.  The Nuclear Receptor LXRα controls the functional specialization of splenic macrophages 
Nature immunology  2013;14(8):831-839.
Macrophages are professional phagocytic cells that orchestrate innate immune responses and display remarkable phenotypic diversity at different anatomical locations. However, the mechanisms that control the heterogeneity of tissue macrophages are not well characterized. Here, we report that the nuclear receptor LXRα is essential for the differentiation of macrophages in the marginal zone (MZ) of the spleen. LXR deficient mice are defective in the generation of MZ and metallophilic macrophages, resulting in abnormal responses to blood-borne antigens. Myeloid specific expression of LXRα or adoptive transfer of wild-type monocytes rescues the MZ microenvironment in LXRα deficient mice. These results demonstrate that LXRα signaling in myeloid cells is crucial for the generation of splenic MZ macrophages and reveal an unprecedented role for a nuclear receptor in the generation of specialized macrophage subsets.
PMCID: PMC3720686  PMID: 23770640
17.  Skeletal muscle Nur77 expression enhances oxidative metabolism and substrate utilization[S] 
Journal of Lipid Research  2012;53(12):2610-2619.
Mitochondrial dysfunction has been implicated in the pathogenesis of type 2 diabetes. Identifying novel regulators of mitochondrial bioenergetics will broaden our understanding of regulatory checkpoints that coordinate complex metabolic pathways. We previously showed that Nur77, an orphan nuclear receptor of the NR4A family, regulates the expression of genes linked to glucose utilization. Here we demonstrate that expression of Nur77 in skeletal muscle also enhances mitochondrial function. We generated MCK-Nur77 transgenic mice that express wild-type Nur77 specifically in skeletal muscle. Nur77-overexpressing muscle had increased abundance of oxidative muscle fibers and mitochondrial DNA content. Transgenic muscle also exhibited enhanced oxidative metabolism, suggestive of increased mitochondrial activity. Metabolomic analysis confirmed that Nur77 transgenic muscle favored fatty acid oxidation over glucose oxidation, mimicking the metabolic profile of fasting. Nur77 expression also improved the intrinsic respiratory capacity of isolated mitochondria, likely due to the increased abundance of complex I of the electron transport chain. These changes in mitochondrial metabolism translated to improved muscle contractile function ex vivo and improved cold tolerance in vivo. Our studies outline a novel role for Nur77 in the regulation of oxidative metabolism and mitochondrial activity in skeletal muscle.
PMCID: PMC3494265  PMID: 23028113
Nr4a; nuclear receptor; mitochondria
18.  Adipose subtype–selective recruitment of TLE3 or Prdm16 by PPARγ specifies lipid-storage versus thermogenic gene programs 
Cell metabolism  2013;17(3):423-435.
Transcriptional effectors of white adipocyte-selective gene expression have not been described. Here we show that TLE3 is a white-selective cofactor that acts reciprocally with the brown-selective cofactor Prdm16 to specify lipid storage and thermogenic gene programs. Occupancy of TLE3 and Prdm16 on certain promoters is mutually exclusive, due to the ability of TLE3 to disrupt the physical interaction between Prdm16 and PPARγ. When expressed at elevated levels in brown fat, TLE3 counters Prdm16, suppressing brown-selective genes and inducing white-selective genes, resulting in impaired fatty acid oxidation and thermogenesis. Conversely, mice lacking TLE3 in adipose tissue show enhanced thermogenesis in inguinal white adipose depots and are protected from age-dependent deterioration of brown adipose tissue function. Our results suggest that the establishment of distinct adipocyte phenotypes with different capacities for thermogenesis and lipid storage is accomplished in part through the cell type–selective recruitment of TLE3 or Prdm16 to key adipocyte target genes.
PMCID: PMC3626567  PMID: 23473036
19.  LXRα is uniquely required for maximal reverse cholesterol transport and atheroprotection in ApoE-deficient mice[S] 
Journal of Lipid Research  2012;53(6):1126-1133.
The liver X receptor (LXR) signaling pathway is an important modulator of atherosclerosis, but the relative importance of the two LXRs in atheroprotection is incompletely understood. We show here that LXRα, the dominant LXR isotype expressed in liver, plays a particularly important role in whole-body sterol homeostasis. In the context of the ApoE−/− background, deletion of LXRα, but not LXRβ, led to prominent increases in atherosclerosis and peripheral cholesterol accumulation. However, combined loss of LXRα and LXRβ on the ApoE−/− background led to an even more severe cholesterol accumulation phenotype compared to LXRα−/−ApoE−/− mice, indicating that LXRβ does contribute to reverse cholesterol transport (RCT) but that this contribution is quantitatively less important than that of LXRα. Unexpectedly, macrophages did not appear to underlie the differential phenotype of LXRα−/−ApoE−/− and LXRβ−/−ApoE−/− mice, as in vitro assays revealed no difference in the efficiency of cholesterol efflux from isolated macrophages. By contrast, in vivo assays of RCT using exogenously labeled macrophages revealed a marked defect in fecal sterol efflux in LXRα−/−ApoE−/− mice. Mechanistically, this defect was linked to a specific requirement for LXRα−/− in the expression of hepatic LXR target genes involved in sterol transport and metabolism. These studies reveal a previously unrecognized requirement for hepatic LXRα for optimal reverse cholesterol transport in mice.
PMCID: PMC3351819  PMID: 22454476
atherosclerosis; nuclear receptor; cholesterol metabolism; apoliporotein
20.  Both K63 and K48 ubiquitin linkages signal lysosomal degradation of the LDL receptor 
Journal of Lipid Research  2013;54(5):1410-1420.
Linkage-specific ubiquitination often leads to distinct cellular events. It has been difficult to establish definitively the requirement for a particular linkage in mammalian degradation pathways due to the inability to deplete endogenous ubiquitin while maintaining cell viability. The E3 ubiquitin ligase inducible degrader of the LDL receptor (IDOL) targets the low density lipoprotein receptor (LDLR) for degradation. The nature of the linkages employed to signal lysosomal degradation of the LDLR, and to signal proteasomal autodegradation of IDOL, have not been determined. We used an inducible RNAi strategy to replace endogenous ubiquitin with mutants lacking K48 or K63. We found that IDOL catalyzes the transfer of ubiquitin chains to itself and to the LDLR that do not contain exclusively K48 or K63 linkages. Thus, LDLR can be targeted to the lysosome by either K48 or K63 linkages. We further demonstrate that although both ubiquitin conjugating enzyme E2 (UBE2)Ds and UBE2N/V1 can catalyze LDLR ubiquitination in a cell-free system, UBE2Ds appear to be the major E2 enzymes employed by IDOL in cells, consistent with their ability to catalyze both K48 and K63 linkages. The results reveal mechanistic insight into the posttranscriptional control of lipoprotein uptake and provide a test of the requirement of linkage-specific ubiquitination for specific lysosomal and proteasomal degradation pathways in mammalian cells.
PMCID: PMC3653405  PMID: 23419260
low density lipoprotein receptor; lysosomal protein degradation; proteasomal protein degradation; E3 ubiquitin ligase
21.  IDOL Stimulates Clathrin-Independent Endocytosis and Multivesicular Body-Mediated Lysosomal Degradation of the Low-Density Lipoprotein Receptor 
Molecular and Cellular Biology  2013;33(8):1503-1514.
The low-density lipoprotein receptor (LDLR) is a critical determinant of plasma cholesterol levels that internalizes lipoprotein cargo via clathrin-mediated endocytosis. Here, we show that the E3 ubiquitin ligase IDOL stimulates a previously unrecognized, clathrin-independent pathway for LDLR internalization. Real-time single-particle tracking and electron microscopy reveal that IDOL is recruited to the plasma membrane by LDLR, promotes LDLR internalization in the absence of clathrin or caveolae, and facilitates LDLR degradation by shuttling it into the multivesicular body (MVB) protein-sorting pathway. The IDOL-dependent degradation pathway is distinct from that mediated by PCSK9 as only IDOL employs ESCRT (endosomal-sorting complex required for transport) complexes to recognize and traffic LDLR to lysosomes. Small interfering RNA (siRNA)-mediated knockdown of ESCRT-0 (HGS) or ESCRT-I (TSG101) components prevents IDOL-mediated LDLR degradation. We further show that USP8 acts downstream of IDOL to deubiquitinate LDLR and that USP8 is required for LDLR entry into the MVB pathway. These results provide key mechanistic insights into an evolutionarily conserved pathway for the control of lipoprotein receptor expression and cellular lipid uptake.
PMCID: PMC3624246  PMID: 23382078
22.  Transcriptional integration of metabolism by the nuclear sterol-activated receptors LXR and FXR 
Nuclear receptors are integrators of hormonal and nutritional signals, mediating changes to metabolic pathways within the body. Given that modulation of lipid and glucose metabolism has been linked to diseases including type 2 diabetes, obesity and atherosclerosis, a greater understanding of pathways that regulate metabolism in physiology and disease is crucial. The liver X receptors (LXRs) and the farnesoid X receptors (FXRs) are activated by oxysterols and bile acids, respectively. Mounting evidence indicates that these nuclear receptors have essential roles, not only in the regulation of cholesterol and bile acid metabolism but also in the integration of sterol, fatty acid and glucose metabolism.
PMCID: PMC3597092  PMID: 22414897
23.  Bone marrow NR4A expression is not a dominant factor in the development of atherosclerosis or macrophage polarization in mice[S] 
Journal of Lipid Research  2013;54(3):806-815.
The formation of the atherosclerotic lesion is a complex process influenced by an array of inflammatory and lipid metabolism pathways. We previously demonstrated that NR4A nuclear receptors are highly induced in macrophages in response to inflammatory stimuli and modulate the expression of genes linked to inflammation in vitro. Here we used mouse genetic models to assess the impact of NR4A expression on atherosclerosis development and macrophage polarization. Transplantation of wild-type, Nur77−/−, or Nor1−/− null hematopoetic precursors into LDL receptor (LDLR)−/− recipient mice led to comparable development of atherosclerotic lesions after high-cholesterol diet. We also observed comparable induction of genes linked to M1 and M2 responses in wild-type and Nur77-null macrophages in response to lipopolysaccharides and interleukin (IL)-4, respectively. In contrast, activation of the nuclear receptor liver X receptor (LXR) strongly suppressed M1 responses, and ablation of signal transductor and activator of transcription 6 (STAT6) strongly suppressed M2 responses. Recent studies have suggested that alterations in levels of Ly6Clo monocytes may be a contributor to inflammation and atherosclerosis. In our study, loss of Nur77, but not Nor1, was associated with decreased abundance of Ly6Clo monocytes, but this change was not correlated with atherosclerotic lesion development. Collectively, our results suggest that alterations in the Ly6Clo monocyte population and bone marrow NR4A expression do not play dominant roles in macrophage polarization or the development of atherosclerosis in mice.
PMCID: PMC3617954  PMID: 23288947
Nur77; Ly6C; nuclear receptor
24.  Reciprocal Metabolic Perturbations in the Adipose Tissue and Liver of GPIHBP1-deficient Mice 
Gpihbp1-deficient mice (Gpihbp1−/−) lack the ability to transport lipoprotein lipase to the capillary lumen, resulting in mislocalization of LPL within tissues, defective lipolysis of triglyceride-rich lipoproteins, and chylomicronemia. We asked whether GPIHBP1 deficiency and mislocalization of catalytically active LPL would alter the composition of triglycerides in adipose tissue or perturb the expression of lipid biosynthetic genes. We also asked whether perturbations in adipose tissue composition and gene expression, if they occur, would be accompanied by reciprocal metabolic changes in the liver.
Methods and Results
The chylomicronemia in Gpihbp1−/− mice was associated with reduced levels of essential fatty acids in adipose tissue triglycerides and increased expression of lipid biosynthetic genes. The liver exhibited the opposite changes—increased levels of essential fatty acids in triglycerides and reduced expression of lipid biosynthetic genes.
Defective lipolysis in Gpihbp1−/− mice causes reciprocal metabolic perturbations in adipose tissue and liver. In adipose tissue, the essential fatty acid content of triglycerides is reduced and lipid biosynthetic gene expression is increased, while the opposite changes occur in the liver.
PMCID: PMC3281771  PMID: 22173228
lipoprotein lipase; hypertriglyceridemia; lipolysis; essential fatty acids; lipid biosynthetic genes
25.  Vestigial-like 3 is an inhibitor of adipocyte differentiation[S] 
Journal of Lipid Research  2013;54(2):473-481.
Adipose differentiation is a complex process controlled by a network of transcription factors and co-regulators. We compared the global gene expression patterns of adipogenic and nonadipogenic clones of 3T3-F442A preadipocytes and identified the transcriptional cofactor, vestigial-like 3 (Vgll3), as an inhibitor of adipogenesis. Vgll3 expression is down-regulated during terminal adipocyte differentiation in vitro and negatively correlates with weight and total fat mass in vivo. Furthermore, enforced Vgll3 expression inhibits the differentiation of preadipocytes in vitro, whereas shRNA-mediated knockdown of Vgll3 expression promotes differentiation. Expression of Vgll3 promoted the expression of genes associated with bone and chondrocyte formation, suggesting that Vgll3 participates in the decision of mesenchymal cells to proceed down the adipocyte, bone, or cartilage lineages. The elucidation of factors involved in specification of the adipocyte phenotype may aid in the identification of new strategies for the treatment of metabolic disease.
PMCID: PMC3541706  PMID: 23152581
mesenchymal differentiation; PPAR; lipid metabolism; nuclear receptor

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